I. Field of the Invention
The present invention relates to equipment employable for detecting the concentration of oxygen in a gas and, more particularly, to a limiting electric current type oxygen sensor with a heater and a limiting electric current type oxygen concentration detecting device having the limiting electric current type oxygen sensor.
The oxygen concentration detecting device in accordance with the present invention can be employed to detect the oxygen concentration of gases exhausted from various types of combustion equipment such as boilers installed in thermal power stations, or internal combustion engines mounted in automobiles.
II. Description of the Prior Art
Needless to emphasize, various types of combustion equipment such as boilers installed in thermal power stations, or internal combustion engines mounted in automobiles are commercially available and contribute to modern life in a variety of applications.
These types of equipment may emit toxic gases in a considerable amount if they are not properly operated. A demand for lower fuel consumption is also strong these days.
It is effective to perform combustion in an atmosphere containing a small amount of fuel with respect to the air content (to be referred to as a "lean" mixture) so as to decrease the toxic gas content of the exhaust gas and to decrease fuel consumption. For example, it is known that the "lean" mixture is used in a Diesel engine. It is also efficient to use the "lean" mixture in a gasoline engine to satisfy the foregoing objects.
However, even in a combustion engine operated with a "lean" mixture, soot may be exhausted to contaminate the atmospheric air and misfiring may occur to exhaust the uncombusted fuel and decrease power or the like when an air-fuel ratio is improperly determined, thus destroying the objective of the combustion engine operated with a "lean" mixture and causing the problems of air pollution and low output power. Therefore, the air-fuel ratio is the most important factor to maximize the combustion efficiency. Generally, in the detection/control technique, the parameter to be controlled (the air-fuel ratio in the "lean" mixture herein) must be detected with high precision and detected at a high speed. In the field of combustion techniques, a proper sensor has not conventionally been proposed. For example, a magnetic oxygen concentration sensor has a poor responsiveness, so that it is not suitably mounted in an automobile. A density type or a thermal conduction type sensor is adversely affected by a small amount of hydrogen (H.sub.2), thus degrading measuring precision. As a result, the conventional sensors have never been suitable for controlling an internal combustion engine.
In order to eliminate the conventional drawbacks, a sensor for detecting an oxygen concentration from a limiting electric current (to be referred to as a limiting electric current type oxygen sensor hereinafter) is proposed. The oxygen sensor of this type has various advantages but yet has a few problems. In a combustion apparatus such as an automobile internal combustion engine, the temperature of the exhaust gas usually changes in accordance with the operating conditions. Therefore, the limiting electric current type oxygen sensor must be operated in a wide temperature range between the low and high temperatures. When the limiting electric current type oxygen sensor is operated at a low temperature, its internal resistance is increased to limit the measurable range of oxygen concentration. Even at a high temperature where the internal resistance is not increased, a problem is still presented in which the relationship between the oxygen concentration and the limiting electric current is slightly changed. However, this problem can be solved when the oxygen sensor is kept at a constant temperature at which the oxygen sensor can be normally operated. A heater is disposed in the vicinity of the oxygen sensor and is driven by a constant voltage source to heat the oxygen sensor in accordance with the conventional method. However, the structure of the oxygen sensor then becomes bulky. Heat conduction is degraded, thus increasing power consumption. As a result, when the flow rate of the gas to be measured varies, the temperature varies accordingly.